This invention relates to multilamp photoflash devices having circuit means for sequentially igniting the flashlamps and, more particularly, to improved switching means for permitting reliable flashing of an array of low voltage filament-type photoflash lamps in rapid succession.
Numerous multilamp photoflash arrangements with various types of sequencing circuits have been described in the prior art, particularly in the past few years. Series and parallel-connected lamp arrays have been shown which are sequentially fired by mechanical switching means, simple electrical circuits, switching circuits using the randomly varied resistance characteristics of the lamps, arc gap arrangements, complex digital electronic switching circuits, light-sensitive switching means and heat-sensitive switching devices which involve melting, fusing or chemical reaction in response to the radiant energy output of a proximate flashed lamp. Examples of previous heat-sensitive switching devices employed in flashlamp arrays are described in the following U.S. Pat. Nos. 3,774,020; 3,726,631; 3,728,068; 3,728,067; 3,714,407; 3,668,421; 3,692,995; 3,666,394; 3,458,270; 3,459,487; 3,459,488; 3,473,880; 3,544,251; 3,443,875; 3,552,896 and 3,562,508.
The present invention is concerned with an improved radiant-energy-activated switching means useful in a relatively inexpensive photoflash unit of the disposable type. In particular, the proposed switching means is particularly advantageous in photoflash arrays employing low voltage filament-type lamps adapted to be ignited sequentially by successively applied low voltage firing pulses from a source such as a camera-shutter-actuated battery switch.
A currently marketed photoflash unit, described in U.S. Pat. No. 3,894,226 and referred to as a flip flash, comprises a planar array of eight high voltage type flashlamps mounted on a printed circuit board with an array of respectively associated reflectors disposed therebetween. The lamps are arranged in two groups of four disposed on the upper and lower halves, respectively, of the rectangular shaped circuit board. A set of terminal contacts at the lower end of the unit is provided for activation of the upper group of lamps, while a set of terminal contacts at the top of the unit is operatively associated with the lower group of lamps. The application of successive high voltage pulses (e.g. 500 to 4,000 volts from, say, a piezoelectric source controlled by the shutter of a camera in which the array is inserted) to the terminal contacts at the lower end of the unit causes four lamps at the upper half of the array to be sequentially ignited. The array may then be turned end for end and again inserted into the camera in order to flash the remaining four lamps.
The flip flash circuit board comprises an insulating sheet of plastic having a conductive circuit pattern, including the terminal contacts, on one side. The flashlamp leads are electrically connected to this circuit by means of eyelets secured to the circuit board and crimped to the lead wires. The circuitry on the board includes six printed, normally open, connect switches, that chemically change from a high to low electrical resistance, so as to become electrically conducting after exposure to radiant heat energy from an ignited flashlamp operatively associated therewith. The purpose of the switches is to promote lamp sequencing and one-at-a-time flashing. The four lamps of each group are arranged in parallel, with three of the four lamps each being connected in series with a respective thermal connect switch. Initially only the first of the group of four lamps is connected directly to the high voltage pulse source. When this first lamp flashes, it causes its associated thermal contact switch (which is series connected with the next, or second, lamp) to become permanently conductive. Because of this action, the second lamp of the group of four is connected to the pulse source. This sequence of events is repeated until all four lamps have been flashed.
A thermal connect switch of the general type discussed above is described in the aforementioned U.S. Pat. No. 3,458,270 of Ganser et. al. as comprising a mass of silver oxide together with a polyvinyl resin binder. Upon exposure to radiant heat, a chemical reaction occurs in which the silver oxide is converted to metallic silver. Unfortunately, such a switch, based entirely on silver oxide, is disadvantageous commercially in that it is unstable under conditions of elevated temperature and high humidity and, consequently, has a limited shelf life. A more recently disclosed switch of this general type is described in U.S. Pat. No. 3,990,833 of Holub et al as being prepared from compositions which impart improved shelf life to the switches under conditions of high relative humidity. More specifically, the improved switch compositions comprise at least seventy percent by weight of a carbon containing silver salt, such as silver carbonate, up to thirty percent by weight of silver oxide, and an organic polymer binder. The patent indicates that the initial resistance of the radiation switch was about 10.sup.10 ohms, and after activation by flashing, the resistance was found to be in the order of 0.1 to 10 ohms. The current source described in the examples for firing the flashlamps was a piezoelectric cell providing a pulse of about 2 kilovolts for a duration of 5 microseconds. Although providing satisfactory switching operation from such high voltage sources, it appears questionable that silver switches of this general type would consistently provide reliable sequential flash operation in an array of filament type flashlamps triggered from a low voltage battery source providing an output pulse of about 2 volts. In such a low voltage array, it would appear that a clean metal to metal contact would be required to provide a reliable electrical connection to the next lamp to be fired.
One of the aforementioned patents, namely, U.S. Pat. No. 3,728,068, describes a similar parallel circuit arrangement in which the heat sensitive switches each comprise a pair of electrical contacts held apart by a chemical or plastic material which melts or otherwise deforms when heated by the radiant output of an ignited flashlamp, thereby permitting the contacts to close and electrically connect the next lamp in the array across the firing pulse terminals. The physical arrangement of these switches is not more specifically defined, however, In another of the aforementioned patents, namely, U.S. Pat. No. 3,562,508, a similar parallel circuit arrangement includes heat sensitive switches comprising a meltable pin passing through a hole in the reflector behind the lamp to hold a resilient switch contact in the open position; upon flashing of an adjacent lamp the pin melts and the switch contacts close to connect the next lamp into the circuit. The switches described in these two patents may overcome the problem of providing a direct metal to metal contact, however, it would appear that the melting or deforming of a fusible pin or other plastic member normally holding the contacts apart could result in a comparatively slow switching action. For camera applications requiring a capability whereby the lamps can be flashed one or two seconds apart, the described switching mechanisms of melting or deforming such fusible members would not appear to provide a sufficiently rapid and fail-safe closed circuit to low voltage pulses.